now it works!I installed mplayer multi thread following the tutorial in this forum and i disabled the option "Switch to TSmuxer when h264 video is PS3 compatible".Is this option important? now i can see mkv without any problem!But the option "Enhanced multicore support" is disabled, and i have a dual core processor, is this correct?

I had the same issue, and it took some time for me to figure out why it happened.

This happens if the hostname of your media server can't be resolved by the DNS that the PS3 uses. Make sure it's added, and it'll work. It was really annoying for me until I figure it out, since the media server application says that teh PS3 has been found, but it can't be seen from the XMB.

great! But i don't understand if i have to configure ps3 or server! I use a static ip with dns provided from my provider and unchecking TsMuxer my problem was solved! I can see Server from xmb and play all files!